In the field of pleasure craft it is known to use small rigid watercraft or inflatable boats provided with an outboard motor, commonly called tenders, as support craft for medium or large craft. Use of the tender allows the crew and the guests of the craft to reach easily any point of the coast that instead might be precluded to the craft due to limits of maneuver, draught or restrictions due to marine orders and the like.
Obviously, these service craft, the tenders, must have the maximum possible space on board yet occupy a minimal amount of space when loaded on the craft during the navigation.
Actually, the presence of an outboard motor (which therefore protrudes with respect to the outline of the tender), the protrusion of the corresponding propeller, which is present even when the boat is not in operation, and the fact that the center of gravity of the assembly is considerably shifted toward the stern (the weight of the motor is very high and concentrated in a cantilever fashion beyond the stern), complicates significantly the operations of launching and recovering the tender. The use of the outboard motor, moreover, entails a considerable reduction in the internal occupiable surface, as a consequence of the need to accommodate lever systems, cables and other apparatuses required for the transmission of the controls of said engine from the respective control bar to a pilot post located in a more advanced position in the boat (abaft). The spaces occupied by these components are obviously not available to passengers.
In order to make hauling operations easier and in order to facilitate the housing of the tender during navigation on the craft that it serves (some crafts have an appropriate compartment for storing the tender), solutions for collapsing the transom of the tender by deflating the buoyancy chambers (watertight chambers which, inflated appropriately, constitute the sides of the boat) and other methods useful for reducing the space occupation of the outboard motor are known (all these solutions are intended to accommodate the motor, during the housing of the boat, on the bottom boards of the boat itself, minimizing overall space occupation).
These specific solutions can be applied only on so-called portable motors, of low power and with tiller steering (every small outboard motor has a tiller with a handpiece that can rotate and constitutes the speed control; the tiller is jointly connected to the outboard motor and its movement turns the motor with respect to the longitudinal axis of the boat).
These solutions for collapsing the motor do not affect the usability of the deck spaces, which remain limited, force various manual operations and are incompatible with remote controls (the cited solutions can be applied only with motors that have tiller steering).
In order to obviate these drawbacks, some shipowners adopt hydrojet tenders, i.e., tenders provided with an inboard motor and a water jet propulsion pump. This solution guarantees to eliminate the problems linked to the weight of the outboard motor cantilevered with respect to the stern and to the additional space occupation with respect to the outline of the watercraft that is typical of outboard motors, but the limitations connected to the use of the hydrojet are the limited space on board for housing personal items and equipment; because much of the space under the seats and under the console is occupied by the motor, the propulsion system and by the respective equipment.
Like all inboard motors, hydrojet motors are particularly complicated in case of maintenance and repair intervention: this is due to poor access to the motor and the poor support network that exists for these motors, which are much less widespread than the outboard units.
An inboard motor on small crafts leads to a high risk of flooding risk of the motor itself, with consequent loss of functionality and serious economic damage.
Because of the poor efficiency of hydrojet propulsion at low speed, one is forced to use motors of high power, weight, cost and complexity in order to obtain a satisfactory performance.
It is therefore evident that hydrojet motors do not solve the typical problems of outboard motors but simply reduce some of the drawbacks of the latter while creating others that in some cases are even worse.
It is noted that a tender should have the maximum people carrying capacity while also guaranteeing maximization of the usable spaces on board (a large boat would be ideal) but at the same time must have the smallest possible space occupation and weight in order to be allow to easily accommodate the tender in the limited available spaces of the main craft. It is fundamental, moreover, to be able to have a forward-shifted center of gravity, because this allows easy launching and hauling especially in yachts equipped with a garage (compartment designed for storing the tender) that use pulling winches and ascent ramps or rollers as a launching and raising apparatus.